Protective radiation shields

Protective radiation shields are used to reduce the radiative exchange between walls at different temperatures thin foils or sheets made of good reflecting materials are placed between the walls, Fig. 5.66. The spaces between the protective shields are normally evacuated so that heat transfer by convection is prevented. This multi-layer arrangement is used predominantly in cryogenic applications for the insulation of containers for very cold liquified gases. 

The heat flux transferred between two very large, parallel, flat walls, according to (5.154) and (5.158), is given by 

We will now consider the case of N radiation shields present between the walls 1 and 2. The emissivity g shall have the same value on both sides of the shield and for all shields. As the shields are very thin, each shield can have a uniform temperature assigned to it. The following equations are obtained with TSi as the temperature of the i-th shield  

The temperatures of the shields drop out of the right hand side when all the 

As can immediately be seen, the heat flux is significantly reduced compared to the case without protective shields (N = 0). Table 5.10 shows examples for s = 0.05, of how the ratio q(N)/q(N = 0) decreases with N for different emissivities = e2 of the outer walls. According to this, the effect of shielding is greater, the higher the emissivity e1 = e2. 

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Radiation methods Radiation protection Radiation shielding... 

Vacuum Radiation Furnaces. Vacuum furnaces are used where the work can be satisfactorily processed only in a vacuum or in a protective atmosphere. Most vacuum furnaces use molybdenum heating elements. Because all heat transfer is by radiation, metal radiation shields ate used to reduce heat transfer to the furnace casing. The casing is water-cooled and a sufficient number of radiation shields between the inner cavity and the casing reduce the heat flow to the casing to a reasonable level. These shields are substitutes for the insulating refractories used in other furnaces. 

In some cases radiation shields are provided to protect against heat effects from fire incidents and operation requirements. The shields usually are of two styles either a dual layer wire mesh screen or a plexy-giass see through barrier. The shields provide a barrier from the effects of radiant heat for specific levels. They are most often used for protection against flare heat and for barriers at fixed firewater monitor devices, most notably at the helidecks of offshore facilities. 

The liquid helium evaporates in the heat exchanger and thus cools dovm the cryopanel. The waste gas which is generated (He) is used in a second heat exchanger to cool the baffle of a thermal radiation shield vi/hich protects the system from thermal radiation coming from the outside. The cold helium exhaust gas ejected by the helium pump is supplied to a helium recovery unit. The temperature at the cryopanels can be controlled by controlling the helium flow. 

Radiation Shielding. Like lead, bismuth absorbs radiation. Therefore, bismuth alloys are widely used in the medical industry during radiation therapy. The alloy is molded to the shape of various organs that are to be shielded. Then the molds are placed between the radiation source and the patient to protect the patient s vital organs from radiation exposure. 

Ozone A molecule made up of three atoms of oxygen. It occurs naturally in the stratosphere and provides a protective layer shielding the Earth from harmful ultraviolet radiation. In the troposphere, it is a chemical oxidant, a greenhouse gas and a major component of photochemical smog. 

In the model pump, a LHe cryopanel (90 cm x 90 cm) made of silver-plated stainless steel is shielded at the rear by a parallel LN2-cooled wall, polished on the panel side and, facing the vacuum system, by a LN2-cooled chevron baffle (with a gas transmission of 20%). The He cryopanel is supplied from a stainless steel LHe reservoir (A = 1.75 m2), wrapped in A1 foil and protected by a LN2 radiation shield of equal area. 

Placing the sensor in a radiation shield without interfering with the fluid flow also reduces the radiation effect. The. sensors of temperature measure men devices used outdoors must be protected from direct sunlight since the radiation effect in that case is sure to reach unacceptable levels. 

Hoe, S.C., Muller, H., Gering, F. Thykier-Nielsen, S., and Sprensen, J.H. (2002) ARGOS 2001 a Decision Support System for Nuclear Emergencies, In Proceedings of the Radiation Protection and Shielding Division Topical Meeting, April 14-17, 2002, Santa Fe, New Mexico, USA. 

Indirect Sintering. A schematic view of a furnace with resistance-heating elements is presented in Fig. 5.34. A modem industrial aggregate is shown in Fig. 5.35. The green compacts (no presintering necessary) are placed inside a cylindrical or basketlike heating element of the furnace (constmcted of Mo or preferably W). In a radial direction to the outside, the furnace is adapted with radiation shields (inner shields made of W, outer sheets made of Mo), which protect the furnace wall and concentrate the heat to... 

Due to the multiple parameters involved in the heat transfer of equipment under radiant heating, it is difficult to recommend threshold levels. It is common practice to position equipment where it cannot be in direct view of the flare flame or provide adequate protection to shield it from the radiation. 

Maximrun radiant heat intensity at any location where urgent emergency action by persormel is required. When personnel enter or work in an area with the potential for radiant heat intensity greater than 2000 Btu/h-fR, then radiation shielding and/or special protective apparel (e.g., a fire approach suit) should be considered. SAFETY PRECAUTION It is important to recognize that personnel with appropriate clothing cannot tolerate thermal radiation at 2000 Btu/h- fE for more than a few seconds. 

Several other important recommendations API 521 makes include restricted access area to personnel, use of radiation shielding, and location of ladders and platforms fhese recommendations are summarized with an illustration in Figure 30.11. The API 521 states that "personnel are commonly protected from high thermal radiation intensity by restricting access to any area where the thermal radiation can exceed 2000 Btu/ hr-fti." Some plants locate fences and warning signs around areas where flare radiation levels can exceed 2000 Btu/hr-fti. 

The API 521 also states that "it is essential that persormel within the restricted area have immediate access to thermal radiation shielding or protective apparel suitable for escape to a safe location." Radiation shields are structures sometimes located near the base of the flare sfack designed wifh a galvanized or reflective steel roof. Some planfs design a roofed corridor that leads from fhe base of fhe sfack to a safe location away from high radiation levels. 

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